As a clean, safe and eco-friendly energy, nuclear power is of great significance to the mitigation of energy safety and global climate change. After the setbacks of the Three Mile Island meltdown and the former Soviet Union's Chernobyl nuclear accident, people are still actively developing a safer and more economical nuclear power generation technology. Currently, the third-generation nuclear power technology has been basically mature.
In the developing fourth-generation nuclear energy system, a high-temperature gas-cooled reactor (HTGR) can achieve a high outlet temperature, high generating efficiency and high-grade heat supply capacity, which has aroused widespread concern.
HTGR adopts ceramic type coated particle fuel elements, uses helium as a coolant and graphite as a moderator. The core outlet temperature may reach 700° C. to 950° C. HTGR is a type of reactor with good safety property due to the following reasons: 1) excellent performance of the fuel elements; 2) large thermal capacity of the graphite core; 3) a full range of negative reactivity temperature coefficient; and 4) the coolant helium being a chemically stable inert gas without phase transition occurring.
The international development of HTGR began in the early 1960, three experimental reactors have been built successively in Britain, Germany and the United States, and two prototype power plants of 330 MW and 300 MW electric power were built and run in the United States and Germany respectively by the 1970s. Without taking any special measures, the maximal core temperature of an early HTGR may exceed 2000° C. under the accident condition of losing coolant, so a dedicated emergency core cooling system is required to prevent overheating damage to the fuel elements.
In order to further improve the safety of reactor, the concept of “modular” high-temperature gas-cooled reactor came into being. The modular HTGR refers specifically to the HTGR with inherent safety characteristics and relatively small single reactor power level. The basic features of such reactor are: under any accident conditions, the residual heat of the reactor core can be discharged through passive way, and the highest temperature of the core fuel would not exceed the allowable limit. Since the possibility of core melt is avoided, even if a beyond design basis accident of very low probability occurs, the radioactive dose outside the nuclear power plant still remains within the limits, the off-site emergency plan does not have to be carried out technically.
Depending on the different shapes of fuel elements, HTGR is classified into the pebble-bed reactor and prismatic reactor. For the former, the coated particle fuels together with the graphite substrate are pressed into fuel pellets of a diameter of 6 cm, to form a flowable pebble bed reactor core and implement the on-load refueling. For the latter, the coated particle fuels together with graphite are pressed into cylindrical pellets, which are then put into a hexagonal prismatic fuel assembly, to form a fixed prismatic core.
Comparing with the prismatic reactor, the pebble-bed HTGR has the following characteristics: 1) on-load handling of fuel elements, high availability rate of power plant; 2) small core excess reactivity, easy reactivity control, high neutron economy; 3) uniform and high discharge burnup, high fuel efficiency; 4) low temperature of fuel particles during normal operation, easy to further enhance the reactor outlet temperature.
As a commercial power plant for on-grid power generation, besides adequate safety, it should possess sufficient competitive economy. The limit of the modular HTGR in the economy mainly comes from safety considerations. The inherent safety of the modular HTGR requires that the decay heat can be discharged from the core by a passive way after the accident, the maximal fuel temperature is ensured to not exceed the design limits, and that the restrictions on the power density and total power of a single core are put forward technically.
How to achieve a better economy under the limit of a small single reactor power has become an issue which must be considered in the processes of design and commercial promotion of the HTGR nuclear power plant.